Iowa Department of Natural Resources, Geological Survey Bureau,
Technical Information Series 32, 1994, p. 29-41


G.R. Hallberg(1), L.S. Seigley(2), R.D. Libra(2), Z-J. Liu(3), R.D. Rowden(2), K.D. Rex(2), M. R. Craig(3), and K.O. Mann(4)


(1) University of Iowa Hygienic Laboratory
University of Iowa; Oakdale Campus
Iowa City, IA 52242

(2) Iowa Department of Natural Resources, Geological Survey Bureau
109 Trowbridge Hall
Iowa City, IA 52242-1319

(3) University of Iowa, Geography Department
Jessup Hall
Iowa City, IA 52242

(4) Department of Geology
Juniata College
Huntingdon, PA 16652


Deducing improvements in water quality related to reductions in nonpoint source pollution (NPS) is a very difficult task. Water quality, at a watershed scale, is a complex integration of many factors upstream and many scales of spatial and temporal variability. Water-quality studies from the Roberts Creek watershed, in the Big Spring groundwater basin, and the Sny Magill and Bloody Run watersheds provide perspectives on this variability for the design of agricultural NPS monitoring. The three watersheds are contiguous and share a similar hydrogeologic framework which allows direct comparison with little confounding climatic/hydrogeologic variability.

The proportion of land in corn production is directly related to the nitrogen loading in these watersheds. For Water Year 1991, 53% of the Roberts Creek watershed was in corn, 3% in forest and/or pasture, and the stream had an annual mean nitrate-N concentration of 9 mg/L. In comparison, the Bloody Run watershed was 39% corn, 30% forest/pasture with mean nitrate-N of 4 mg/L; the Sny Magill watershed was 26% corn, 49% forest/pasture with a mean nitrate-N of 2 mg/L. In all three streams nitrate-N concentrations decline downstream related to in-stream biological processing. The rate (and mass) of in-stream nitrate removal varies seasonally, reaching a maximum during summer low-flow periods. With the pronounced downstream depletion of nitrate, water quality from small headwater basins cannot be directly compared with larger aggregate watersheds.

Through the Big Spring Basin Demonstration Project significant improvements in nitrogen input efficiency have occurred; fertilizer-N rates for continuous corn have been reduced from 178 to 137 lbs/ac from 1981 to 1991. These reductions have yet to be definitively reflected in groundwater discharged at Big Spring because of extremes in rainfall, recharge, and overall water flux. Nitrate-N concentrations declined to an all-time low in 1989, during a drought, followed by nitrate-N increases to record highs in 1990 and 1991. These trends were not unexpected: the dry conditions allowed the accumulation, or storage of residual nitrate-N in the soil-water system, which was subsequently mobilized by excessive precipitation and recharge.

With the length of record now available in the Big Spring basin, the changes in landuse and N-loading resulting from the Payment-In-Kind (PIK) set-aside program in 1983 can provide an overall model of results expected at the watershed scale. The reduction in corn acreage under PIK, coupled with the changes in nitrogen management, reduced basin N-loading about 27%. With an apparent two-year time lag, this reduction in loading was reflected by a significant change in groundwater quality. Nitrate-N concentrations in groundwater in Water Year 1985 declined about 28%, dropping from annual flow-weighted means of 10.2 mg/L in Water Year 1983 and 9.6 in Water Year 1984, to 6.9 mg/L in Water Year 1985; in Water Year 1986 nitrate-N increased back to 9.6 mg/L. However, groundwater discharge and nitrate-N concentrations are significantly related. Hence, because discharge also declined across this period a decline in nitrate-N could be expected, as well. Only with several years of data subsequent to Water Year 1985 has it been possible to accomplish analysis that provides statistical, as well as intuitive confidence that these water-quality improvements in Water Year 1985 are related to landuse and management changes. Even with over a decade of water-quality data, the Big Spring basin demonstrates the complexity of assessing water-quality improvements related to improved nitrogen management.